A protective coating layer is formed on the surface of the workpiece in the plastic working of metals with the goal of preventing galling by avoiding direct metal-to-metal contact between the workpiece and tool. Various protective coating layers have been used to date. One method in general use involves the formation of, for example, an oil film, soap film, metal soap film, or wax film, either as such or in combination with a binder component. Another widespread method comprises the formation of a coating of a lubricating component on a reactive conversion coating layer (for example, a phosphate coating or oxalate coating) already formed on the metal surface. In the case of the former method, the protective coating layer formed directly on the workpiece surface not only prevents direct metal-to-metal contact, but through its lubricating properties also reduces the coefficient of friction of the workpiece surface. This enables a reduction in the working energy through a relaxation of the load on the protective film layer itself and a relaxation of heat production by the working process. The protective coating can be formed in this technology by dissolving or dispersing the lubricating component in water, either by itself or together with a binder component as necessary or desired, and coating and drying the resulting bath on the workpiece surface. This process therefore offers the advantages of a low number of process steps and easy bath management. However, within the sphere of the severe working sector with its high degree of working, the protective coating layer is unable to follow the enlargement in the surface area of the workpiece and an acceptable performance by the protective film is frequently not secured due to an extreme thinning of the film as well as the generation of open areas in the film.
In the case of the latter method, direct contact between the tool and workpiece is avoided through the formation of a fine and dense reactive conversion coating layer on the workpiece surface. A coating of lubricating component is generally also placed on the surface of this conversion coating. Because the adherence and retention of the lubricating component layer is excellent in this case due to the surface roughness, this technology can also be used in severe working environments since the surface enlargement due to working can be satisfactorily followed. However, the conversion coating is elaborated by a chemical reaction, which necessary entails a complex procedure for managing the treatment bath and a large number of steps in addition to a high cost when capital and wastewater treatment expenses are included. In addition, the chemical reactivity varies substantially as a function of the target material, and the application of conversion treatment to a conversion-resistant, weakly reactive material in particular stands little chance of success.
In order to solve the problems identified above, efforts have been made to improve the properties of the protective films afforded by the former method up to a level equivalent to that of the protective films afforded by conversion treatment. These efforts have resulted in the introduction of methods that use oil-based lubricants and methods that use water-based lubricants. Within the sphere of the oil-based lubricants, Japanese Published (Examined or Kokoku or B) Patent Application Number Hei 4-1798 (1,798/1992) discloses a “cold-working lubricant afforded by blending a metal soap or solid lubricant into a lubricating oil comprising a blend of extreme-pressure agent (e.g., chlorinated paraffin, phosphate ester), isobutylene/n-butene copolymer, and animal or vegetable oil”. While this is a high-performance lubricant, it is nevertheless associated with several problems: it provides a workability somewhat poorer than that provided when lubrication is effected by carrying out a reactive soap lubrication treatment on top of a conversion coating treatment, and it generates an unpleasant odor during the working process due to its use of the extreme-pressure additive.
The water-based lubricants can be used directly in a wet process or can be used as dried coatings in a dry process. Water-based lubricants that are used directly in a wet process are, like the aforementioned oil-based lubricants, directly flowed onto the workpiece or tool. In the case of water-based lubricants that are used as dried coatings, a solid coating is obtained, just as for the aforementioned conversion coating, by immersion in a treatment bath followed by evaporation of the water fraction in a drying step. Japanese Published (Examined or Kokoku or B) Patent Application Number Sho 58-30358 (30,358/1983) discloses a water-based lubricant of the first type in the form of a “lubricant for the hot-working of metal tubing, comprising a bicarbonate (solid) main component to which small amounts of dispersant, surfactant, and solid lubricant have been added”. This lubricant, however, has not yet achieved widespread use in place of conversion coating treatments. An example of the second type of water-based lubricant is disclosed in Japanese Laid-Open (Unexamined or Kokai or A) Patent Application Number Sho 52-20967 (20,967/1977) in the form of a “lubricant composition comprising solid lubricant, a conversion coating-forming agent, and a base of water-soluble polymer or water-based emulsion thereof”. This lubricant, however, does not match conversion coating treatments.
More recently, Japanese Laid-Open (Unexamined or Kokai or A) Patent Application Number 2000-63680 has disclosed a lubricating agent composition for the plastic working of metals that contains synthetic resin and water-soluble inorganic salt in specific proportions. This lubricating agent composition prevents direct metal-to-metal contact with the tool through the formation of a coating comprising the synthetic resin and water-soluble inorganic salt uniformly precipitated on the workpiece surface. In addition, the presence in the coating of a lubricating component in a freely selected proportion provides a performance at least as good as that afforded by the formation of a lubricating component layer on a phosphate coating. In the case of the composition under discussion, however, a single coating composed of the aforementioned chemicals carries the dual functions of galling resistance and lubricity. As a consequence, coating defects and extreme differences in the quantity adhered arising from uneven add-on during, for example, the coating process, can easily become starting points for the occurrence of galling. Since this is a fatal flaw, coating uniformity becomes a crucial feature, yet this consideration has received no attention.